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IC 


9000 



Bureau of Mines Information Circular/1984 




Overspeed Protection for Mine Diesels 



A Literature Review 



By Lito C. Mejia and Robert W. Waytulonis 




UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular 9000 



w 



^ 



u 



v\ \ Vei S Vii-W ^ « 'feu^reA.U o V ^^ v w^e fe J 



Overspeed Protection for Mine Diesels 

A Literature Review 

By Lite C. Mejia and Robert W. Waytulonis 




UNITED STATES DEPARTMENT OF THE INTERIOR 
William P. Clark, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 



Library of Congress Cataloging in Publication Data: 






Mejia, Lito C 

Overspeed protection for mine diesels. 

(Information circular / United States Department of the Interior, Bu- 
reau of Mines ; 9000) 

Bibliography: p. 12-13. 

Supt. of Docs, no.: I 28.27:9000. 

1. Mining machinery— Safety measures. 2. Diesel motor— Safety 
measures. 3. Governors (Machinery). 4. Methane. I. Waytulonis, 
Robert W. II. Title. III. Series: Information circular (United States. 
Bureau of Mines) ; 9000. 



TNaeSrtM [TN345] 622s [622'. 2] 84-600201 



^ 



CONTENTS 

<^ Page 
o^ 

\^ Abstract 1 

Introduction 2 

Mine accident data 2 

J. Compression ignition of methane in a diesel engine 3 

^ Effects of various methane concentrations and engine conditions 3 

J Duel-fuel engine studies 5 

N^) Fuel governors 6 

Types of governors and principles of operation 6 

, Special conditions 8 

p^ Positive-shutdown devices 9 

vi^ Summary 11 

^ References 12 

ILLUSTRATIONS 

1. Autolgnltion of methane-air corrected for Induction heating by normalizing 

on 8 .8 vol pet methane 3 

2. Engine speed versus methane concentration. 5 

3. Typical diesel fuel-governor operational characteristics curves 7 



-5^ 





UNIT OF MEASURE ABBREVIATIONS USED IN 


THIS REPORT 


cm^/min 


cubic centimeter per minute pet 


percent 


»F 


degree Fahrenheit rpm 


revolution per minute 


hp 


horsepower s 


second 


mln 


minute vol pet 


volume percent 



OVERSPEED PROTECTION FOR MINE DIESELS 
A Literature Review 
By Lito C. Mejia and Robert W. Waytulonis 



ABSTRACT 

Diesel-powered equipment operating in a gassy underground mine could 
conceivably ingest a methane-air mixture from the mine atmosphere that 
could cause the diesel engine to overspeed, or exceed its rated speed. 
Engine overspeed, particularly if extreme (engine runaway), could re- 
sult in personal injury, a possible mine explosion, and/or catastrophic 
engine failure. In this report, the Bureau of Mines reviews the liter- 
ature on the potential hazards of me thane -induced diesel engine over- 
speed. Also included are the Bureau's findings from consultations with 
representatives of the diesel engine industry to determine the specific 
engine behavior that could be expected under the overspeed condition. 
The report summarizes data on mine accidents involving methane, dis- 
cusses results of tests on the compression ignition of methane in die- 
sel engines , and examines fuel governors and intake-air-cutoff devices , 
two kinds of devices used to prevent overspeed. 

Analysis of the information gathered suggests that the conditions 
necessary to cause a diesel engine to overspeed uncontrollably are not 
likely to occur, particularly if the engine is equipped with flame- 
proofing devices. The available literature and lack of actual over- 
speed case histories suggest that the safety devices currently in use 
are sufficient to prevent diesel engine overspeed. 



^Mechanical engineer. 
^Supervisory physical scientist. 
Twin Cities Research Center, Bureau of Mines, Minneapolis, MM. 



INTRODUCTION 



In 1976, 135 mobile diesel units were 
in use in 18 underground coal mines and 
366 such units were in use in 11 gassy 
metal and nonmetal mines O).^ Currently, 
mobile diesel units are used in approxi- 
mately 82 underground coal mines (a more 
than fourfold increase since 1976) and 15 
MNM mines (2^). Since 1976 there has been 
a sevenfold increase in the number of 
diesels in use in coal mines , and the 
number has almost doubled in gassy MNM 
mines. About 1,000 diesel units operate 
in coal mines, and another 621 operate in 
underground gassy MNM mines (2^). A gassy 
classification by the Mine Safety and 
Health Administration (MSHA) means either 
a flammable gas has been ignited or a 
concentration of 0,25 vol pet or more of 
flammable gas such as methane has been 
detected in the atmosphere of any open 
working area (_3) . All coal mines are 
gassy. 

Engine surface and exhaust temperatures 
are limited for diesels approved under 
30 CFR 36 O ) . Dieselized equipment ap- 
proved under this regulation is referred 
to as permissible equipment. Not all 
diesel units operating in gassy mines are 
permissible since only dieselized units 
that are to be operated at or near an ac- 
tive working area (within the last open 
crosscut) require approval. Thus, the 
number of diesel units operating in gassy 
MNM and coal mines is a combination of 
permissible and nonpermissible equipment. 
It is not known at this time how many of 



the total number of units are permissible 
units and how many are nonpermissible, 
Dieselized equipment can be operated if 
the concentration of methane in the mine 
air is less than 1.0 vol pet in any ac- 
tive working area (30 CFR 57) (^) . 

A diesel engine operating in a gassy 
underground mine could possibly encounter 
a methane-air mixture of sufficient com- 
bustion energy to accelerate the engine 
uncontrollably. The ingestion and com- 
bustion of the fuel-air mixture could 
produce engine power that is not under 
the equipment operator's control, a con- 
dition that could result in engine run- 
away (speeds greater than 150 pet of the 
rated engine speed) and catastrophic en- 
gine failure due to excessive inertial 
loads (_5) , 

The increasing use of diesel equipment 
in mine working areas where combustible 
methane-air mixtures may be present has 
prompted this study of the potential haz- 
ards of methane-induced overspeed and the 
precautionary measures that have been 
taken to prevent its occurrence. It is 
stipulated in 30 CFR 36.23 (3) that the 
intake system of an approved diesel en- 
gine operating in a gassy atmosphere must 
include a valve, operable from the opera- 
tor's compartment, to shut off the air 
supply. The question remains, however, 
whether this precaution is sufficient to 
prevent potential overs'peed and/or run- 
away conditions upon methane ingestion. 



MINE ACCIDENT DATA 



Methane concentrations greater than 
1,0 pet occasionally occur in mines, as 
shown in the records of mine explosions 
(^-_7) , The approximate flammability 
range wherein an ignition or explosion 
is possible is 5 to 15 pet methane in 
air (8;"9^) . The autoignition temperature 
of firedamp, or mine methane, is about 

■^Underlined numbers in parentheses re- 
fer to items in the list of references at 
the end of this report. 



1,049° F, as compared to 1,000° F for 
commercial-grade methane ( 10 ) . 

From 1941 to 1969, 740 methane igni- 
tions or explosions occurred in coal 
mines, and since 1969 another 690 have 
been recorded (_7) , indicating dilution to 
less than 1.0 vol pet methane in the mine 
air is not always maintained. These data 
are approximate, since some minor explo- 
sions go unreported. About 85 pet of the 
incidents since 1969 occurred at the face 



and were caused by friction from the 
cutting bits of continuous miners. The 
other 15 pet were caused by miscellaneous 
sources such as weld sparks, overheated 



roof-drill bit, blasting, etc. No re- 
cords of mine explosions attributable to 
diesel engines ingesting methane were 
discovered. 



COMPRESSION IGNITION OF METHANE IN A DIESEL ENGINE 



EFFECTS OF VARIOUS METHANE 
CONCENTRATIONS AND ENGINE CONDITIONS 

Compressed ignition of methane-air mix- 
tures was investigated by the Canada Cen- 
ter for Mineral and Energy Technology 
(CANMET) to determine the possibility of 
diesel engines continuing to operate on a 
methane-air mixture after diesel fuel 
delivery has been shut off by the fuel- 
injection pump (11), The engine used for 
this study was a four-stroke single- 
cylinder liquid-cooled test engine with 
variable-compression ignition and indi- 
rect injection; the engine was rotated 
at 900 rpm by a constant-speed motor. 
Audible knock determined whether the in- 
jested methane-air mixture was igniting 
in the cylinder (11-12) , Knock is a 
characteristic violent detonation pro- 
cess associated with methane-air mixtures 
(low-cetane, high-octane rating) at high 
engine compression ratios (_5, 13-16), 
The test procedure was to set the engine 
compression ratio and methane ingestion 
rate, run the engine on diesel fuel for 2 
min (to reach thermal equilibrium), and 
follow immediately with 15 s of methane- 
air ingestion without diesel fuel. If 
the methane ignited on almost every com- 
pression stroke for more than 15 s, con- 
tinuous combustion was said to have 
occurred. Several different methane in- 
gestion rates were used to determine the 
minimum continuous-combustion compression 
ratios of four combinations of engine in- 
take and coolant temperatures. 

The test results indicate that under 
certain conditions, high-compression 
indirect-injection diesel engines can 
operate on ingested methane-air mixtures. 
Figure 1 shows the compression ratio re- 
quired for autoignition at various meth- 
ane concentrations. It is apparent that 
an 8,8-vol-pct methane concentration 
(nearly stoichiometric) is most sensitive 



to compression ignition and that the hot- 
ter the engine, the lower are the com- 
pression ratios required to autoignite 
the methane-air mixtures. 

The CANMET study also suggested that 
glowing combustion chamber deposits could 
act as an ignition source and allow 
the engine to ignite methane at lower 
compression ratios. In addition, fuel 
leakage — even a small volume of fuel nor- 
mally insufficient to maintain idling in 
plain air — could result in a dual-fuel 
operation, with the engine continuing to 



28 



26 



24 



O 22 



20 



16 




KEY 



Symbol 


Intake air 


Engine coolant 


• 


83° F 


90° F 


o 


88° F 


2I5°F 


D 


92° F 


2I5°F 


A 


I46°F 


2I5°F 


A 


IOO°F 


352°F 



5 7 9 II 13 15 

APPROXIMATE INGESTED METHANE CONCENTRATION, vol pet 

FIGURE 1. - Autoignition of methane-air cor- 
rected for induction heating by normalizing on 
8.8 vol pet methane. 



operate on methane combustion Initiated 
by the compression ignition of the diesel 
fuel. The CANMET study also indicated 
that in larger engines, compression igni- 
tion of methane can occur at lower com- 
pression ratios because the greater com- 
bustion volume to wall area ratio results 
in lower compression-heat losses, CANMET 
recommended that quick-closing intake 
valves continue to be required on flame- 
proof diesel-powered equipment certified 
for use in coal and other gassy under- 
ground mines. 

The occurrence of continuous combustion 
in the CANMET study indicated that under 
certain conditions a high-compression 
diesel engine can operate on an ingested 
methane-air mixture. This knocking comr- 
bustion was also observed by the South- 
west Research Institute (SwRI), which 
found it to be insufficient to success- 
fully power a test engine O ) . SwRI per- 
formed tests sponsored by MSHA to evalu- 
ate the potential hazard of a diesel 
ingesting combustible methane-air mix- 
tures. The types of diesels used in un- 
derground mines are usually rated near 
2,100 rpm, and previous SwRI experience 
had shown that engine speeds greater than 
3,500 rpm can cause engine disintegration 
due to excessive inertial loads, (That 
is, the engine becomes mechanically over- 
stressed due to the inertia of its rotat- 
ing parts.) 

SwRI's experimental design used a sin- 
gle-cylinder test engine with a compres- 
sion ratio of 18:1 coupled to an eddy- 
current dynamometer and direct-current 
motor. The test procedure involved run- 
ning the engine at constant speed, at a 
sufficient diesel fuel rate to produce 
a measurable torque. Commercial-grade 
natural gas was introduced into the in- 
take duct in increasing concentrations 
until the methane-air mixture was too 
rich to sustain combustion. Testing was 
repeated at engine speeds of 1,000, 2,000 
and 3,000 rpm. Collected data were used 
to calculate the volumetric and thermal 
efficiency of the engine and the methane 
fuel-air ratio. 



The test procedure required the use of 
diesel fuel as an ignition source for the 
methane-air mixtures. Attempts to run 
the engine on methane alone were unsuc- 
cessful. With the engine hot, ignition 
of methane without diesel fuel could be 
accomplished, but severe knocking re- 
sulted. In this experiment, the combus- 
tion process produced insufficient power 
for runaway. 

Owing to its specific properties, espe- 
cially its low propensity to ignite under 
compression, methane is a poor substitute 
for diesel fuel. Theoretically, the min- 
imum ignition temperature of industrial 
methane (1,000° F) requires an adiabatic 
compression ratio of approximately 19:1 
(at an intake temperature of 60° F) (17) , 
and the previously discussed CANMET test 
results support this. This ratio, 19:1, 
is typical of present mine diesels, but 
the quality and quantity of fuel, and ac- 
tual nonadiabatic conditions, affect the 
true compression ratio necessary for 
ignition. Firedamp has an ignition tem- 
perature of 1,100° F (10), due to its 
relative impurity. Therefore, the com- 
pression ratio necessary for the ignition 
of firedamp will be higher. 

The SwRI investigators also estimated 
the thermal efficiency of methane and 
used this value to calculate the addi- 
tional power produced by the combustion 
of methane. The power contributed by the 
diesel fuel was subtracted from the ac- 
tual power measured. The engine was not 
able to operate on methane alone, so the 
thermal efficiency of methane was not 
readily calculated. When methane is in- 
gested, the engine will accelerate until 
the power produced by the methane is 
exactly balanced by the power required 
to overcome engine friction. For the 
calculation of a runaway condition, the 
methane-produced power was equated to the 
engine friction. The engine-power equa- 
tion used was based on the calculated 
methane thermal efficiency and the engine 
volumetric efficiency. Engine friction 
data from Taylor ( 15 ) were used in 
the calculations. For each methane-air 



ratio, the equation was solved for the 
engine speed at which methane-produced 
power equaled friction power. 

Plotting engine speed versus the meth- 
ane-air ratio gave the methane concen- 
tration range at which runaway speeds 
(greater than 3,500 rpm) could be at- 
tained. As shown In figure 2, this range 
Is about 3,5 to 15 vol pet. The steep 
slopes In this figure clearly show engine 
speed to be highly sensitive to the meth- 
ane concentration. Given an Ignition 
source, the theoretical results of meth- 
ane Ingestion for an unloaded engine 
would be Instantaneous acceleration to 
very high and destructive speeds. Where 
an engine In a vehicle moves across a 
nonhomogeneous methane-air mixture, er- 
ratic engine behavior and severe vehicle 
surging could result since the transmis- 
sion would be engaged. 

Actual testing never proceeded beyond 
running the engine at 3,000 rpm. Running 
the engine at this speed collapsed the 



7,000 



5,000 



uj 4,000 



^ 3,000 - 



1,000 




4 6 8 10 

METHANE, vol pet 



FIGURE 2. • Engine speed versus methane 
concentration. 



piston crown and cracked the cylinder 
head, terminating the experiment. This 
failure was attributed to the knocking 
characteristics (loss of combustion con- 
trol) that are observable in methane com- 
bustion when compression ratios are high 
(15). 

The SwRI investigators concluded from 
their experiments that runaway can occur 
only under abnormal engine conditions and 
only if an ignition source is present. 
If an ignition source is present, runaway 
can only occur if the methane concentra- 
tion range is between 3,5 to 15 vol pet. 
Also, the most likely result of methane 
ingestion will be knocking combustion 
producing insufficient power to cause 
runaway , 

SwRI's unsuccessful attempts to operate 
its diesel test engine on methane alone 
was in agreement with a similar experi- 
ment conducted by Deutz Corp, , a manufac- 
turer of diesel engines in the Federal 
Republic of Germany (FRG) , As a result 
of Bureau Inquiries to Deutz, tests were 
performed on the manufacturer's dual-fuel 
engines in a small-scale experiment. A 
dual-fuel engine is a diesel engine that 
operates on natural gas combustion initi- 
ated by the compression ignition of a 
small amount of diesel fuel, 

DUEL-FUEL ENGINE STUDIES 

Testing at the Deutz R&D Center in 
Porz, FRG, defined the minimum value of 
diesel fuel needed to support combustion 
(18). The testing procedure Involved 
bringing the engine to operating equilib- 
rium and decreasing the amount of diesel 
fuel that was mixed with natural gas un- 
til the engine stopped. The dual-fuel 
engine tested had a compression ratio of 
17:1. It was shown that the Deutz dual- 
fuel diesel engines required a minimum of 
15 pet diesel fuel mixed with natural gas 
to operate. Natural gas by itself did 
not support combustion under the diesel 
cycle (18) . It was also discovered that 
the Injector tip (a possible ignition 
source Implied by CANMET and SwRI) can 
overheat if too little diesel fuel is in- 
jected. A hot Injector can serve as an 



ignition source for the natural gas, but 
testing showed that even with an over- 
heated injector tip, diesel fuel was 
still required to sustain combustion. To 
avoid injector-tip overheating, which 
would necessitate the costly replacement 
of burned-off injector tips, Deutz recom- 
mends using a minimum of 25 pet diesel 
fuel for its dual-fuel engines. 

A number of dual-fuel combustion stud- 
ies are reported in the literature. The 
difficulty of operating a diesel engine 
solely on a methane-air mixture has been 
well documented O, j3 , 18-19) . Although 
methane was found to be a very suitable 
primary fuel, it requires, as do other 
gaseous fuels, an independent ignition 
source owing to its poor self-ignition 
quality (high-octane, low-cetane rating) 



(13, 19). Only with pilot (diesel) fuel 
injection was it possible to run a diesel 
engine with a high-octane fuel as the 
main energy source ( 19 ) . Most high- 
octane gaseous fuels require about 30 pet 
(on an energy basis) pilot-fuel injection 
(19) . For methane, as much as 43 pet 
pilot fuel was needed to achieve optimum 
engine efficiency (19) . As rediscovered 
by SwRI and Deutz, both the performance 
and use of dual-fuel engines are limited 
by knock, a characteristic loss of com- 
bustion control especially with the igni- 
tion of methane at high compression 
ratios ( 13 , 15 , 19 ) . Knock is the com- 
bustion process experienced by CANMET in 
the motoring action of its test engine 
(11-12) . Knocking combustion has limited 
the normal operation of dual-fuel engines 
(5, 13, 18-19). 



FUEL GOVERNORS^ 



The fuel governor, a part found on 
every diesel engine, controls speed and 
torque by regulating the fuel flow rate. 
Because the engine has no control over 
the contents of the intake air, and the 
combustion of methane in the engine in- 
creases power and speed, the fuel gover- 
nor acts to compensate for the speed and 
power fluctuations caused by methane in 
the intake air. How several types of 
fuel governors regulate the fuel flow and 
engine speed is summarized below. 

In order to maintain desired speeds un- 
der variable loads, as in mobile diesel- 
powered mining equipment, the fuel rate 
must be metered to correspond to the re- 
quired torque. In diesel engines, fuel 
metering is achieved by changing the 

'*This section includes considerable 
information from technical and product 
literature from American Bosch Div. of 
United Technologies, Springfield, MA; 
Barber-Coleman Co., Rockford, IL; Cater- 
pillar Co., Peoria, IL; Detroit Diesel 
Div, of General Motors Corp,, Detroit, 
MI; Hoof Products, Chicago, IL; LUCAS- 
CAV, London, England; Robert Bosch GmbH, 
Stuttgart, FRG; Terex Div, of General Mo- 
tors Corp., Detroit, MI; and Woodward 
Governor Co,, Fort Collins, CO, 



quantity of fuel injected by movement of 
the fuel-injection pump's control rod. 
Control-rod movement is accomplished by 
the acceleration pedal or by a signal 
from an engine revolution-per-minute 
sensor, 

TYPES OF GOVERNORS AND PRINCIPLES 
OF OPERATIONS 

Four basic types of governors are com- 
mercially available: mechanical, pneu- 
matic, electric, and hydraulic (20-22) . 
The mechanical governor is a speed-sensi- 
tive control that uses tbe movement of 
flyweights under the influence of centri- 
fugal force. The pneumatic governor uses 
a diaphragm activated by the vacuum pres- 
ent in the engine's intake manifold. 
Where constant-speed (isochronous) opera- 
tion is required, electric governors are 
used. They are activated by engine-speed 
signals obtained from a magnetic pickup 
that monitors the teeth of the flyweel's 
ring gear. Off-speed is corrected elec- 
trically through the magnetic pickup sen- 
sor and linkage to the fuel system. Like 
mechanical governors, hydraulic governors 
depend on the variation of centrifugal 
force created by flyweights in the gover- 
nor. However, this force does not di- 
rectly operate the fuel-control mechanism 



as in the mechanical governor. Instead 
the fuel rod is connected to a piston- 
type pilot valve that controls the flow 
of high-pressure oil to a cylinder-type 
servomotor. 

Two kinds of governors are commonly 
used in mobile mining diesels: minimum- 
maximum and variable-speed governors (_5) . 
Both are mechanical governors. Typical 
operational characteristics of a vari- 
able-speed governor are shown in figure 
3. Speed control is exerted over the en- 
tire engine speed-load range. The curves 
represent various throttle and fuel-cut- 
off positions for given horsepowers and 
speeds; they also represent the amount of 
fuel injected at fixed accelerator pedal 



positions. With the pedal position con- 
stant, methane ingestion increases the 
engine horsepower and thus engine speed; 
and the governor compensates by reducing 
the diesel fuel flow rate, as indicated 
by the downward curves of figure 3. At 
any pedal position, a further increase in 
engine speed will result in the fuel be- 
ing shut off. Downhill travel or some 
other reduction in engine load will re- 
duce the amount of diesel fuel. If the 
reduction in load is significant enough, 
continued increases in engine speed will 
cause the fuel to be shut off completely 
(23). 

A minimum-maximum governor exerts speed 
control only near the idle speed and at a 




1,000 



1,500 



2,500 



3,000 



2,000 
ENGINE SPEED, rpm 

FIGURE 3. - Typical diesel fueUgovernor operational characteristics curves (23). Downward curves 
represent governor actiono 



maximum setting (usually the rated 
speed) . In an engine operating at a con- 
stant load and speed (constant pedal po- 
sition) , a sudden load decrease will 
cause the engine to accelerate up to the 
maximum governed speed. Only then will 
the governor react to reduce the rate of 
fuel flow. This surging will normally be 
countered by the operator easing off the 
accelerator pedal. An increase in engine 
speed from idle or from the maximum rated 
speed (no load) will cut off the diesel 
fuel (2, 20-22). 

In principle, governor control allows 
enough fuel to be injected so the engine 
can attain its rated speed. If a rated 
speed of 2,100 rpm is exceeded, for exam- 
ple, the flyweights override a prese- 
lected spring force and move the fuel 
rack linkage to reduce the rate of in- 
jected fuel. For an engine running at 
its rated speed under zero load, only 
enough fuel is injected to overcome 
internal engine friction, A further in- 
crease in engine speed (either by motor- 
ing or ingestion of a combustible meth- 
ane-air mixture) to above the rated speed 
moves the linkage to a position that al- 
lows no fuel to be injected. Even fur- 
ther engine speed increases will continue 
to move the linkage but will have no 
practical effect because no fuel is being 
injected (24) . 

For a loaded engine, methane in the in- 
take air will produce additional power 
(19, 25-26) , The operator or fuel gover- 
nor will reduce the fuel to maintain the 
injection-rate power level, or the speed 
will increase until the maximum speed 
regulation described above becomes ac- 
tive. In effect, the engine is operating 
at a lower-than-noinnal throttle position, 
A sudden load reduction, such as a clutch 
disengagement, will cause the engine to 
accelerate. Fuel reduction and speed 
stabilization will occur in the zero-load 
condition previously described, 

Diesel engines accelerate rapidly, and 
some "overshoot" can be expected. With 
methane-air ingestion, engine speeds to 
2,500 rpm can occur before stabilization 
is achieved. Acceleration from idle to 



zero-load maximum speed can take less 
than 1 s, and stabilization at the maxi- 
mum zero-load speed can occur within an 
additional 2 s (24). 

Mechanical governors react quickly un- 
der changing load, and the response to a 
change in engine speed is immediate, A 
typical response time for a change in en- 
gine load is less than Is, Some vari- 
able-speed governors, if subjected to in- 
stantaneous speed change, have a typical 
response time of about 0,05 s to cut back 
fuel delivery (27) , This response is in- 
dependent of load. Mechanical fuel gov- 
ernors are rated to regulate within 0,5 
to 1 pet of the rated engine speed ( 28 ) . 
It is estimated that one model of mechan- 
ical governor would reestablish engine 
speed within 0.2 to 0.3 s, even with 
methane ingested ( 15 ) . In applications 
where electric governors are used, speed 
control within 0.2 pet of the selected 
speed is common ( 28 ) . For a mining en- 
gine encountering a gassy air mixture, 
the mechanical governor will provide al- 
most instantaneous control, 

SPECIAL CONDITIONS 

Because diesel fuel or some other igni- 
tion source is required to initiate com- 
bustion of methane-air mixtures , and be- 
cause of the governor's quick reaction 
time (to cut off diesel fuel if the rated 
speed is exceeded) , it is improbable that 
destructive speeds could be attained ( 23 , 
25 , 28-29), There are, however, specific 
engine and governor conditions that could 
alter this assessment. 

The possibility of fuel remaining in 
the fuel-injection system after shutoff 
was brought up by governor and engine 
manufacturers (^3, 25.* 2:2)* ^^^^ ^^® 
governor in the full cutoff position, 
small amounts of fuel can still be in- 
jected into the combustion chamber by 
residual pressure. Normally, however, 
these injections are not adequate to sus- 
tain combustion (23), For one flyweight- 
type governor, leakage can occur (at 
about 35 cm^/min) at the complete cutoff 
position (29) . It is not known how much 
fuel actually gets into the combustion 



chamber. When an electric governor is 
used, the failsafe mode returns the fuel 
throttle activator to a "minimum fuel" 
position, but this minimum fuel position 
is not necessarily a complete cutoff 
condition. 

Another consideration is the role of 
the governor as a speed-controlling de- 
vice and not as an engine-shutdown de- 
vice. When diesel fuel is cutoff and a 
condition exists where combustion of 
methane delivers torque greater than that 
required for the existing load, a violent 
instability with the governor cycling 
from full on to full off at the system 
natural frequency is possible. This cy- 
cle would continue unless there is an in- 
dependent overspeed shutdown ( 28 ) . The 
severity of this cyclic response can be 
undesirable, depending on methane concen- 
tration, engine conditions, and the iner- 
tial forces attained. Stable speed could 
not be achieved, since the governor has 
no control over the energy source. If 
the methane did not provide sufficient 
additional power to deliver the torque 
required for the existing load, speed 
stability might be achieved, because some 
control would be exerted by the governor 
(30). 

In effect, a diesel operating in a gas- 
sy mine will behave as a dual-fuel en- 
gine. The most likely ignition source 
for the methane-air mixture is the avail- 
able fuel supplied by the engine's fuel- 
injection system. However, other engine 
conditions that potentially could provide 
an ignition source for methane (5, 11, 



18) include: (1) injectors getting stuck 
open as a result of a gummed-up fuel sys- 
tem; (2) bad enhaust valves — improperly 
seating valves cannot perform the neces- 
sary heat transfer to the head (which 
will erode the valve or seat and provide 
a hot spot for ignition); (3) carbon de- 
posits providing a "glow plug" effect; 
(4) an overheated engine caused by poor 
cooling; (5) a badly worn engine which 
consumes lubrication oil; (6) an over- 
turned engine or an engine operating at 
a large tilt causing lube oil consump- 
tion; (7) overfilling of the oil-bath air 
cleaner; (8) failed turbocharger bearing 
seals, with leaking lube oil ingested 
through the intake; and (9) abnormally 
high compression ratios — for example, a 
high-altitude engine with high-compres- 
sion pistons operated at or near sea 
level . 

The above conditions cannot be con- 
trolled by a properly working governor. 
A malfunctioning governor can fail to 
properly regulate the quantity of diesel 
fuel being injected into the engine. The 
governor, though simple in principle, is 
complex owing to its many parts. Mis- 
adjustment or failure of any part may 
take the following forms: (1) flj^eights 
"frozen" because of insufficient lubri- 
cation or excessive bearing-pin or gear 
wear, (2) improperly adjusted set screws, 

(3) broken throttle rods and stops, 

(4) fatigued inner springs, (5) excessive 
friction in linkages, (6) engine parts 
rubbing on throttle rod, (7) slipping 
drive belts, or (8) pilot-valve plungers 
sticking because of dirt in the channels. 



POSITIVE-SHUTDOWN DEVICES 5 



Several overspeed and emergency intake- 
air-shutdown protective devices are 

-*This section includes considerable in- 
formation from technical and product lit- 
erature from A^DT Controls, Richmond, CA; 
Detroit Diesel Div. of General Motors 
Corp., Detroit, MI; Progress Equipment 
Co., Inc., Houston, TX; Pyroban Ltd., 
Sussex, England; and Special Products 
Div, of Otis Engineering Corp., Dallas, 
TX. 



available for diesels used in underground 
gassy mines. All of these are based on 
the premise that either the governor can 
fail or combustible fuel from an uncon- 
trolled source can self-ignite during the 
diesel cycle. If methane were to ignite 
under compression, or if the fuel system 
is not failsafe, protection would be pro- 
vided with an air shutoff system. 

The most common air~shutoff device is 
an air flapper valve. Upon release of a 



10 



latching mechanism, a thin steel plate 
covers the air inlet. An electronic 
speed switch (a speed-sensing device) can 
be used to trigger the latching mecha- 
nism. Another device uses a spring- 
loaded poppet valve to block the air 
inlet. This valve is entirely self-con- 
tained and does not need an external 
power source. It is fitted upstream of 
the air-intake manifold and actuated by 
an air-pressure differential. Closure 
occurs when the pressure difference 
across the valve becomes great enough to 
overcome the spring tension, which is set 
at the desired overspeed limit. Little 
or no modification is needed to adapt the 
design to a diesel's air-intake manifold. 

Other types of air-control devices are 
maintained in the open position by an ac- 
tuator supplied with engine oil at normal 
pressure through an orifice. In this 
design, oil pressure keeps three valves 
closed. Should the exhaust-gas tempera- 
ture, coolant temperature, or engine 
speed exceed preset limits, one of the 
valves would open and return oil to the 
engine. With the loss of oil pressure, 
the actuator would close the air valve 
and stop the engine. Another triggering 
device is an electronic impulse that 
operates an actuator in a cylinder. A 
spring-loaded release mechanism, trig- 
gered by a gas-operated cylinder, closes 
the air valve. Another approach to keep- 
ing air out of the engine is to use (CO2) 
to displace intake air. The inert CO2 
prevents combustion in the chamber. 

The air-shutoff devices described above 
are activated by certain engine condi- 
tions such as overspeed, high tempera- 
ture, etc. Use of a methane monitor 
could be another triggering method for 
air shutoff and may be appropriate where 
diesel operate in gassy atmospheres. The 
monitor would be set at the 1.0-pct 
methane-in-air concentration specified in 
30 CFR 57. In noncoal mines, this regu- 
lation prohibits the operation of diesel 
equipment if the methane concentration 
exceeds 1.0 pet in any active working 
area. The methane monitor could be used 
to simultaneously shut off air and fuel 



as an additional safeguard to the normal 
governor shutoff mechanism. 

Use of positive-shutdown devices on 
diesel engines operating in hazardous 
areas where gassy air mixtures may occur 
is a common practice in industry and is 
required by MSHA in underground mines. 
Current U.S. regulations require the use 
of a manual air shutoff that is operable 
from the operator's compartment (_3 ) . In 
Canadian underground coal mines, mobile 
diesels are required to have a manually 
operated air-shutoff valve that automati- 
cally stops the engine after the actua- 
tion of fuel shutoff in the safety shut- 
down system (31). In the United Kingdom 
(U.K.), the approval of diesel-powered 
equipment used in underground mines re- 
quires the air-inlet systems to control 
the effct of methane in the intake air 
on the engine's ability to continue to 
run after the fuel supply has been cut 
off (32). Where an air-shutoff valve is 
fitted, it must automatically cut off the 
air supply to stop the engine if the en- 
gine speed exceeds the governed speed by 
more than 20 pet ( 32 ) . 

Recognition of the potential danger of 
diesels operating in hazardous areas is 
also found in other industries ( 10 , 33). 
Incidents in the petroleum and petrochem- 
ical industries have shown that diesel 
engines can provide a source of ignition 
for flammable vapors and can also create 
a hazard by overspeeding as a consequence 
of ingesting of these vapors (33). In 
order to provide a consistent approach in 
formulating safety requirements for the 
abatement of such hazards , the Oil Compa- 
nies Materials Association (OCMA) has 
developed recommendations for the pro- 
tection of diesel engines used in poten- 
tially hazardous areas. Those who pre- 
pared these recommendations considered 
the regulations of the U.K. Safety in 
Mines Research Establishment (SMRE) and 
those of the French and West German 
Governments ( 33 ) . Current practices and 
intentions of U.K. petroleum and petro- 
chemical companies were also considered. 
OCMA recommends the use of a manual air- 
shutoff device as sufficient for abating 



11 



the hazards of flammable vapor inges- 
tion by an attended engine but con- 
siders an automatic device to be neces- 
sary for unattended machines (33). This 



recommendation represents a standard of 
good practice and exists as mandatory in 
the regulations and/or codes of practice 
in other countries. 



SUMMARY 



Conceivably, a diesel engine operating 
in a gassy underground mine could ingest 
a methane-air mixture from the mine at- 
mosphere that could cause the engine 
speed to increase, resulting in engine 
overspeed or runaway. 

High-speed diesel engines such as those 
used in mobile mining equipment are gov- 
erned not to overspeed (exceed the rated 
engine speed) under any load condition. 
The maximum operating speed for these 
diesels is approximately 150 pet of the 
rated engine speed; at higher (runaway) 
speeds, excessive inertial loads will 
destroy the engine. The speed-limiting 
device used in these engines, a mech- 
anical-type fuel governor, is designed to 
maintain a desired operating speed as 
well as completely shut off injected die- 
sel fuel in the event of overspeed. Com- 
plete fuel shutoff occurs when the rated 
engine speed is exceeded by more than 20 
pet. The normal time between a signifi- 
cant change in engine load and the gover- 
nor's response to maintain the balance 
speed or stop diesel fuel injection is 
nearly instantaneous. 

The fuel governor plays a critical role 
in controlling a possible overspeed con- 
dition. At the onset of methane inges- 
tion, combustion of the gas-air mixture 
is initiated by the compression ignition 
of the diesel fuel supplied in the normal 
manner. Even if the governor is success- 
ful at cutting off the diesel fuel flow, 
the question remains whether the methane- 
air mixture would autoignite under the 
diesel cycle — without the aid of pilot 
(diesel) fuel injection — and continue to 
run the engine to destructive speeds. 

Methane makes a poor fuel for diesel 
engines owing to its low compression- 
ignition quality (as indicated by cetane 
nimber) and will not normally ignite un- 
less the compression ratio is high or an 



ignition source is present. The diffi- 
culty of methane combustion through com- 
pression ignition has been well demon- 
strated in single-cylinder engine tests 
and duel-fuel engine experiments, Diesel 
engines will not operate on methane alone 
under normal conditions. Even with a hot 
injector tip as an ignition source, one 
dual-fuel engine still required a small 
amount of diesel fuel to initiate combus- 
tion. Motoring tests on a variable- 
compression-ratio engine produced contin- 
uous knocking combustion at compression 
ratios in the range of mining diesels, 
but this was achieved at the most sensi- 
tive methane concentration (stoichiomet- 
ric) and at abnormally hot engine condi- 
tions. Other tests have determined that 
this knocking combustion causes some en- 
gine damage, but insufficient power to 
produce runaway. 

Considering only the possibility of 
diesel fuel ignition of the combustible 
mixture, an uncontrolled engine is im- 
probable. However, undesirable engine 
behavior can result from the cyclic be- 
havior in which the governor tries to 
maintain the balance speed, leakage from 
remaining fuel in the injection system 
after fuel shutoff, and the extent of 
overspeed. There are also potentially 
dangerous engine conditions that could 
sustain methane combustion. Although 
such conditions are abnormal, the engine 
could achieve destructive speeds because 
the fuel governor, under certain condi- 
tions, has no control over the ignition 
source, A malfunctioning governor would 
worsen the problem. 

Mobile machinery receive much abuse in 
the mining industry and poorly maintained 
engines are not uncommon. The engine's 
fuel governor does not have complete con- 
trol over the effects of combustible 
methane-air mixtures that could be en- 
countered in underground gassy mines. 



12 



Requirement of air shutoff systems for 
diesels operating in hazardous areas 
where gassy-air mixtures may occur is 
currently a readily accepted practice. 
Potential engine damage, personal in- 
jury and possible mine explosions all 
prescribe an air shutoff system as a 
reasonable requirement. The available 



literature and the lack of documented ac- 
cidents suggest that a manual device is 
sufficient control for the potential haz- 
ards of methane ingestion by an engine in 
an attended vehicle with a trained opera- 
tor. For an unattended engine an auto- 
matic air shutoff system should be used. 



REFERENCES 



1. LeFranc, C. I. (MSHA) . Private 
communication, 1981; available upon re- 
quest from L. C. Mejia, BuMines, Minne- 
apolis, MN. 

2. Brash, J. K. (MSHA). Private com- 
munication, 1983; available upon request 
f rom L. C. Mejia, BuMines, Minneapolis, 
MN. 

3. U.S. Code of Federal Regulations. 
Title 30 — Mineral Resources; Chapter 1 — 
MSHA, Department of Labor; Subchapter E — 
Mechanical Equipment for Mines; Tests for 
Permissibility and Suitability; Fees; 
Part 36 — Mobile Diesel-Powered Transpor- 
tation Equipment for Gassy Noncoal Mines 
and Tunnels; Section 36.2; and Subchapter 
N — Metal and Nonmetal Mine Safety and 
Health; Part 57 — Safety and Health Stan- 
dards-Metal and Nonmetal Underground 
Mines; Section 57.21-lc; July 1, 1982. 

4. U.S. Code of Federal Regulations. 
Title 30 — Mineral Resources; Chapter 1 — 
MSHA, Department of Labor; Subchapter N — 
Metal and Nonmetal Mine Safety and 
Health; Part 57 — Safety and Health Stan- 
dards-Metal and Nonmetal Underground 
Mines; Sections 57.21-24; July 1, 1982. 

5. Wood, C. Effects of Ambient Meth- 
ane on Diesel Engines. SW Res. Inst., 
San Antonio, TX, final rep., project 11- 
5452-029, July 1980, 31 pp. 

6. Nagy, J. The Explosion Hazard in 
Mining. Dep. of Labor Inf. Rep. 1119, 
1981, 69 pp. 

7. U.S. Mine Safety and Health Admini- 
stration. Methane Outburst Reports. Of- 
fice Inf., MSHA, Arlington, VA, 1969-80. 



8. Rose, J. W. , and J. R. Cooper. 
Technical Data on Fuel. Wiley, 1977, 
343 pp. 

9. Zabetakis, M. G. Flammability 
Characteristics of Combustible Gases and 
Vapors. BuMines B 627, 1965, 121 pp. 

10. Tyrer, P. G. Flammable Gases and 
the Diesel Engine. Publication 401 pres. 
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request from L. C. Mejia, BuMines, Minne- 
apolis, MN. 

11. Stewart, D. B. , and N. N. Kallio. 
Auto-Ignition of Methane-Air in a Diesel 
Engine. Canada Center for Miner, and En- 
ergy Technol. (CANMET), Rep. ERD/MRL 76- 
113 (R), Aug. 1976, 9 pp. 

12. Stewart, D. B. , J. P. Morgan, and 
E. D. Dainty. Canada Center for Miner, 
and Energy Technol. (CANMET), Rep. ERP/ 
MRL 77-89 (OP), Aug. 1977, 10 pp. 

13. Karim, G. A. The Ignition of a 
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Fuel Spray Injection With Reference to 
Dual-Fuel Combustion. Pres. at Natl, 
Fuels and Lubricants Meeting, Tulsa, OK, 
Oct. 29-31, 1968. SAE preprint 680768, 
12 pp. 

14. Obert, E. F. Internal Combustion 
Engines. Intext Educational Publ., 3d 
ed., 1973, 740 pp. 

15. Taylor, C. F. The Internal Com- 
bustion Engine in Theory and Practice, 
MIT Press, v. 2, 1966, 783 pp. 



13 



16. Rogowski, A, R, Elements of In- 
ternal Combustion Engines, McGraw-Hill, 
1953, 205 pp. 

17. Stinson, K. W. Diesel Engineering 
Handbook. Business Journals, Inc., 12th 
ed. , 1972, 333 pp. 

18. Smith, A. (Deutz Corp.). Private 
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quest from L. C. Mejia, BuMines, Minne- 
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19. Bro, K. , and L. S. Pederson. Al- 
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Passenger Car Meeting, Detroit Plaza, De- 
troit, MI, Sept. 26-30, 1977, SAE pre- 
print 770794, 16 pp. 

20. Straubel, M. The Robert Bosch In- 
Line Pump for Diesel Engines. Pres. at 
MECCA Of f -Highway Vehicle Meeting and Ex- 
position, Milwaukee, WI, Sept. 10-13, 
1979, SAE preprint 790901, 11 pp. 

21. Bosch Company. Technical Instruc- 
tion: Fuel Injection Equipment for Die- 
sel Engines, Governors or In-Line Pumps. 
Robert Bosch GmBH, Dep. for Tech. Publ. 
(Stuttgart, Federal Republic of Germany), 
1975, 48 pp. 

22. Adey, A. J., F. Cunliffe, and 
J. E. Mardell. Rotary Fuel Injection 
Pump Developments for High Speed Diesel 
Engines. Pres. at Internat. Cong, and 
Exposition, Co bo Hall, Detroit, MI, Feb. 
23-27, 1981, SAE preprint 810516, 10 pp. 

23. Wing, M. K. (LUCAS-CAV, Lucas In- 
dustries, Inc.). Private communication, 
1981; available upon request from L. C. 
Mejia, BuMines, Minneapolis, MN. 

24. Baugh, E. D. (Detroit Diesel Al- 
lison). Private communication, 1981; 
available upon request from L. C. Mejia, 
BuMines, Minneapolis, MN. 

25. Sallee, J. (Caterpillar Co., En- 
gine Div.), Private communication, 1981; 
available upon request from L. C. Mejia, 
BuMines, Minneapolis, MN. 



26. Mertens, H. , and H. S. Bochum. 
(Exhaust Behavior on Diesel Engines Where 
Intake Air Contains Methane.) Final rep. 
dev. proj . Inst. Mech. Eng. and Inst. 
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kasse, Gluckauf (Eng. Transl.) v. 115, 
No. 11, 1979, 4 pp. 

27. Gross, R. N. (United Technologies 
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28. Wilson, K. (Dynalco Corp.). Pri- 
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request from L. C. Mejia, BuMines, Minne- 
apolis, MN. 

29. Finkbiner, K. B. (Cummins Engine 
Co.). Private communication, 1981; 
available upon request from L. C. Mejia, 
BuMines, Minneapolis, MN. 

30. Wimp, J. W. (Woodward Governor 
Co.). Private communication, 1981; 
available upon request from L. C. Mejia, 
BuMines, Minneapolis, MN. 

31. Dainty, E. D. , and J. P. Morgan. 
The Certification of Flameproof Diesel- 
Powered, Rubber-Tired Trackless, Self 
Propelled Vehicles for Use in Underground 
Coal Mines in Canada. Canada Center for 
Miner, and Energy Technnol. (CANMET), 
Rep. ERP/MRL 79-68 (TR) , July 1979, 
83 pp. 

32. Health and Safety Executive (Lon- 
don, England). Test and Approval of 
Diesel and Storage Battery Powered Loco- 
motives and Trackless Vehicles and Die- 
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in Mines. Test. Memo. 12., 1977, 39 pp. 

33. Oil Companies Materials Associa- 
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of Diesel Engines in Hazardous Areas. 
Heyden and Sons, Ltd. (London), publ. 
MEC-1, 1977, 14 pp. 



ftU.S. CPO: 1984-505-019/5070 



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